Method for the production of liquefied natural gas and liquid nitrogen

ABSTRACT

A method for the production of liquefied natural gas and liquid nitrogen is provided. The method may include providing a high pressure natural gas stream, splitting the high pressure natural gas stream into a first portion and a second portion, and liquefying the first portion of the high pressure natural gas stream to produce an LNG stream. The refrigeration needed for cooling and liquefaction of the natural gas and liquefaction of the nitrogen can be provided by a nitrogen refrigeration cycle and letdown of the second portion of the high pressure natural gas stream.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 62/201,947, filed on Aug. 6, 2015, U.S. Provisional PatentApplication No. 62/305,381, filed on Mar. 8, 2016, and U.S. ProvisionalApplication Ser. No. 62/370,953 filed on Aug. 4, 2016, all of which arehereby incorporated by reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to a method for efficientlyproducing liquefied natural gas (LNG).

BACKGROUND OF THE INVENTION

Many locations utilize a high pressure (transmission) network and alower pressure (distribution) network to supply natural gas through alocal area. The transmission network typically acts as a freeway toeconomically send the gas over long distances to the general area, whilethe distribution network acts as the roads to send the gas to theindividual users within a local area. Pressures of these networks varyby location, but typical values are between 30-80 bara for transmissionand 3-20 bara for distribution. Some applications (e.g., cogeneration,boilers, etc. . . . ) have high flowrates of natural gas and otherutilities such as nitrogen which are letdown to the consumer or to thelower pressure network at relatively constant flow, pressure andtemperature conditions. This pressure letdown energy is often notutilized.

Traditionally natural gas is compressed and sent down pipelines underhigh pressure to transport the gas to market. High pressures are used inorder to reduce the volumetric flow of the gas thereby reducing pipediameters (capex) and/or compression energy related to pressure losses(opex). Pipeline operators also utilize the high pressure as a buffer toaccommodate transient demands. When the gas has arrived at its usepoint, the pressure of the natural gas is reduced in one or more controlvalves to its final pressure for consumption. The available energy fromthe reduction in pressure of the natural gas is wasted in the controlvalves as well as any chilling effect (also known as the Joule Thomsoneffect) caused by the flow of natural gas through these devices. Suchsystems often require heaters and condensate systems due to the colderconditions of the downstream gas.

In the past, advantage has been taken of this wasted energy byfacilities utilizing the energy and refrigeration effect of expandingthe natural gas. One such facility was designed and constructed by AircoIndustrial Gases' Cryoplants Division in the early 1970's in Reading,Pa. for UGI Corporation. It employed a natural gas pressure reductionstation (“Letdown Station”) to make liquefied natural gas (“LNG”) orliquid nitrogen (“LIN”). A majority of the natural gas entering theplant under high pressure from the transportation pipeline was cooledand sent to an expansion turbine where energy and refrigeration weregenerated. The remainder of the stream was subsequently cooled with therefrigeration and a portion liquefied. The liquefied portion was thenpassed to a storage tank as LNG product. The natural gas that was notliquefied was warmed, collected and sent to the low pressure main at alower pressure than the high-pressure main.

U.S. Pat. No. 6,196,021 describes a system that uses natural gasexpansion to provide refrigeration to liquefy a natural gas stream whichis then vaporized by heat exchange with a nitrogen stream to cool thenitrogen stream. This refrigeration supplements refrigeration providedby nitrogen pressure letdown and a nitrogen cycle to provide liquidnitrogen. Similarly, U.S. Pat. No. 6,131,407 describes a system thatproduces LIN to be sent directly to an air separation unit (“ASU”) toassist refrigeration of the ASU. U.S. Patent Application Publication No.2014/0352353 describes a similar system to the system of disclosed byU.S. Pat. No. 6,131,407, but adds that the LIN produced can be sent to atank instead of being used to liquid assist the ASU. In each of thesesystems, LNG is revaporized to provide for nitrogen cooling. However, itis not desirable to liquefy and then revaporize the natural gas, as thisis thermally inefficient. U.S. Pat. No. 6,694,774 describes a systemthat uses natural gas letdown to provide refrigeration to produce aliquefied natural gas stream, where the refrigeration is supplemented bya closed loop mixed refrigerant cycle.

FIG. 1 provides a process flow diagram for a typical small LNG schemethat utilizes a nitrogen cycle 50, which includes nitrogen compressor10, coolers 11, 21, 26, and first and second turbine boosters 20, 25, ina closed loop. For purposes discussed herein, a turbine booster is acombination of a turbine and a booster, in which the booster is powered,at least partially, by the turbine, which is typically accomplished viaa common shaft. Natural gas 2 is first purified of components that woulddamage equipment or freeze during liquefaction in purification unit 30.Purified natural gas 4 is then cooled in heat exchanger 40, where it iscondensed into LNG 6 using refrigeration provided by the nitrogenrefrigeration cycle 50. Typically, heavy hydrocarbons (pentane andheavier) are removed from the natural gas either before or from anintermediate location of the exchanger 40 by adsorption, distillation orgas-liquid separator in order to prevent these components from freezingin the exchanger 40. In the example of FIG. 1 the natural gas 4 iswithdrawn from an intermediate section of the heat exchanger 40 in orderto remove the heavy hydrocarbons 8 using gas liquid separator 5. In thetypical setup shown of FIG. 1, the power required to produce 342 mtd ofLNG is approximately 7155 kW, meaning the specific power of this setupis approximately 502 kWh/mt.

Therefore, it would be advantageous to provide a method and apparatusthat operated in a more efficient manner yielding a lower cost of LNG.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus thatsatisfies at least one of these needs. In certain embodiments, theinvention can provide a lower cost, more efficient and flexible methodto produce LNG. For example, in certain embodiments, the invention canalso include coproduction of liquid nitrogen (“LIN”). In additionalembodiments, the invention may include varying the production rates ofeither or both the LIN and LNG, based on power costs, product demand,and/or supply levels.

Nitrogen is transported through high pressure pipelines because of thelower transport cost of reduced volumetric flows associated with highpressure gas. Typically such pipelines operate in the range of 30 to 50bara. Customers using nitrogen from a pipeline often do not need thenitrogen at these pressures. For example, nitrogen is typically used asan inert utility fluid at pressures in range of 3 to 8 bara. As such, inthese locations, potential refrigeration capacity is wasted.Additionally, there are instances in which producers of the nitrogen gasfeeding the pipeline do not operate at 100% of equipment designcapacity, and therefore, large nitrogen compressors are either notoperating or not operating at optimum capacity. This can occur if thedemand for nitrogen is lower than originally anticipated, for example.Another reason this occurs is because the nitrogen producing equipmentis sized to meet peak customer demand under peak operating scenarios,ambient conditions, catalyst life, and the like. As such, the nitrogenproducing equipment may be designed to be underutilized during manyoperating scenarios when other systems are not able to accommodateincreased loads.

In certain embodiments of the invention, a process can provide for LNGand/or LIN production with at least reduced energy input by using therefrigeration capabilities of letdown of natural gas and let down ofnitrogen or a gas rich in nitrogen. An example of a gas rich in nitrogenis a lean synthetic air stream with less than 12% O₂ (e.g., due to thelimit of combustion for a mixture with methane). In embodiments, theletdown process occurs at a location that is proximate to an existingfacility or location where the letdown of both natural gas and nitrogenoccurs to serve the needs of the facility, such that LNG and/or LIN canbe produced with reduced operating costs and/or capital costs ascompared to a situation without the benefit of the letdown of a gasstream (e.g., a nitrogen stream, a stream of gas rich in nitrogen, or anatural gas or other high pressure gas stream at a production site).

In one embodiment, the invention can include a method for the productionof liquefied natural gas (“LNG”). In one embodiment, the method caninclude the steps of: a) providing a nitrogen refrigeration cycle,wherein the nitrogen refrigeration cycle is configured to providerefrigeration within a heat exchanger; b) purifying a first natural gasstream in a first purification unit to remove a first set of impuritiesto produce a purified first natural gas stream; c) cooling andliquefying the first natural gas stream in the heat exchanger using therefrigeration from the nitrogen refrigeration cycle to produce an LNGstream, wherein the first natural gas stream has an LNG refrigerationrequirement, wherein the LNG stream is liquefied at a first pressureP_(H); d) purifying a second natural gas stream in a second purificationunit to remove a second set of impurities to produce a purified secondnatural gas stream; e) partially cooling the second natural gas streamin the heat exchanger; f) withdrawing the partially cooled secondnatural gas stream from an intermediate section of the heat exchanger;g) expanding the partially cooled second natural gas stream to a mediumpressure P_(M) in a natural gas expansion turbine to form a cold naturalgas stream, wherein the medium pressure P_(M) is at a pressure lowerthan the first pressure P_(H); and h) warming the cold natural gasstream in the heat exchanger by heat exchange against the first naturalgas stream to produce a warm natural gas stream at the warm end of theheat exchanger, wherein the natural gas expansion turbine drives a firstbooster, wherein the LNG refrigeration requirement is supplied by acombination of refrigeration from the nitrogen refrigeration cycle andstep h).

In optional embodiments of the method for the production of LNG:

-   -   the first booster is configured to compress the second natural        gas stream or a stream derived from the second natural gas        stream;    -   the first booster is configured to compress a stream selected        from the group consisting of the first natural gas stream, the        first purified natural gas stream, the second natural gas        stream, the purified second natural gas stream, the partially        cooled natural gas stream, the warm natural gas stream, and a        nitrogen fluid within the nitrogen refrigeration cycle;    -   the first set of impurities has a freezing point at or above the        liquefaction temperature of methane at the first pressure P_(H);    -   the second set of impurities comprises water;    -   the nitrogen refrigeration cycle comprises a recycle compressor,        a turbine, a booster and a plurality of coolers, wherein the        turbine and booster are configured such that the turbine is        configured to power the booster;    -   the first natural gas stream and the second natural gas stream        come from the same natural gas source;    -   the natural gas source is a natural gas pipeline having a        pressure between 15 and 100 bara;    -   the first natural gas stream comes from a first natural gas        source, and the second natural gas stream comes from a second        natural gas source, wherein the first and second natural gas        sources are different sources;    -   the first natural gas source comprises a natural gas pipeline;    -   the natural gas pipeline has a pressure between 15 and 100 bara;    -   the first purification unit and the second purification unit are        the same unit; and/or    -   the first purification unit and the second purification unit are        separate units, wherein the first purification unit is        configured to remove at least water and carbon dioxide, and        wherein the second purification unit is configured to remove at        least water.

In another aspect of the invention, a method for the production ofliquefied natural gas (“LNG”) is provided. In this embodiment, themethod comprising the steps of: a) providing a nitrogen refrigerationcycle; b) cooling and liquefying a first natural gas stream in a heatexchanger by heat exchange with nitrogen from the nitrogen refrigerationcycle to produce an LNG stream, wherein the LNG stream is liquefied at afirst pressure; c) expanding a second natural gas stream to a secondpressure to produce an expanded natural gas stream; and d) warming theexpanded natural gas stream in the heat exchanger to produce a warmednatural gas stream, wherein step d) provides a portion of therefrigeration used to cool and liquefy the first natural gas stream.

In optional embodiments of the method for the production of LNG:

-   -   the first natural gas stream comes from a first natural gas        source, and the second natural gas stream comes from a second        natural gas source, wherein the first and second natural gas        sources are different sources; and/or    -   the first natural gas liquefied in step b) is derived from the        expanded natural gas stream, wherein the first pressure and the        second pressure are about the same.

In another aspect of the invention, a method for the production ofliquefied natural gas (“LNG”) is provided. In this embodiment, themethod comprising the steps of a) providing a high pressure natural gasstream; b) splitting the high pressure natural gas stream into a firstportion and a second portion; c) cooling and liquefying the firstportion of the high pressure natural gas stream to produce an LNGstream; d) providing a first portion of refrigeration via a nitrogenrefrigeration cycle, wherein the nitrogen refrigeration cycle comprisesa recycle compressor, a turbine, a booster and a plurality of coolers,wherein the turbine and booster are configured such that the turbine isconfigured to power the booster; e) providing a second portion ofrefrigeration by expanding the second portion of the high pressurenatural gas; and f) using the first portion of refrigeration and thesecond portion of refrigeration to achieve the cooling and liquefactionof the first portion of the high pressure natural gas stream in step c).

In another aspect of the invention, a method for the production ofliquefied natural gas (“LNG”) and liquid nitrogen (“LIN”) is provided.In this embodiment, the method can include the steps of: a) providing anitrogen refrigeration cycle, wherein the nitrogen refrigeration cycleis configured to provide refrigeration within a heat exchanger, whereina portion of the nitrogen within the nitrogen refrigeration cycle iswithdrawn and liquefied yielding a liquid nitrogen product, wherein atleast an equal portion of gaseous nitrogen is introduced to the nitrogenrefrigeration cycle as is withdrawn; b) purifying a first natural gasstream in a first purification unit to remove a first set of impuritiesto produce a purified first natural gas stream; c) cooling andliquefying the first natural gas stream in the heat exchanger using therefrigeration from the nitrogen refrigeration cycle to produce an LNGstream, wherein the first natural gas stream has an LNG refrigerationrequirement, wherein the LNG stream is liquefied at a first pressureP_(H); d) purifying a second natural gas stream in a second purificationunit to remove a second set of impurities to produce a purified secondnatural gas stream; e) partially cooling the second natural gas streamin the heat exchanger; f) withdrawing the partially cooled secondnatural gas stream from an intermediate section of the heat exchanger;g) expanding the partially cooled second natural gas stream to a mediumpressure P_(M) in a natural gas expansion turbine to form a cold naturalgas stream, wherein the medium pressure P_(M) is at a pressure lowerthan the first pressure P_(H); and h) warming the cold natural gasstream in the heat exchanger by heat exchange against the first naturalgas stream to produce a warm natural gas stream at the warm end of theheat exchanger, wherein the natural gas expansion turbine drives a firstbooster, wherein the LNG refrigeration requirement is supplied by acombination of refrigeration from the nitrogen refrigeration cycle andstep h).

In optional embodiments of the method for the production of LNG and LIN:

-   -   the first booster is configured to compress the second natural        gas stream or a stream derived from the second natural gas        stream;    -   the first booster is configured to compress a stream selected        from the group consisting of the first natural gas stream, the        purified first natural gas stream; the second natural gas        stream, the purified second natural gas stream, the partially        cooled natural gas stream, the warm natural gas stream, and a        nitrogen fluid within the nitrogen refrigeration cycle;    -   the liquid nitrogen product has a LIN refrigeration requirement,        wherein the LIN refrigeration requirement is supplied by a        combination of refrigeration from the nitrogen refrigeration        cycle and step h);    -   the first set of impurities has a freezing point at or above the        liquefaction temperature of methane at the first pressure P_(H);    -   the second set of impurities comprises water;    -   the nitrogen refrigeration cycle comprises a recycle compressor,        a turbine, a booster and a plurality of coolers, wherein the        turbine and booster are configured such that the turbine is        configured to power the booster;    -   the nitrogen refrigeration cycle further comprises a nitrogen        feed compressor;    -   the first natural gas stream and the second natural gas stream        come from the same natural gas source;    -   the natural gas source is a natural gas pipeline having a        pressure between 15 and 100 bara;    -   the first natural gas stream comes from a first natural gas        source, and the second natural gas stream comes from a second        natural gas source, wherein the first and second natural gas        sources are different sources;    -   the first natural gas source comprises a natural gas pipeline;    -   the natural gas pipeline has a pressure between 15 and 100 bara;    -   the first purification unit and the second purification unit are        the same unit;    -   the first purification unit and the second purification unit are        separate units, wherein the first purification unit is        configured to remove at least water and carbon dioxide, and        wherein the second purification unit is configured to remove at        least water; and/or    -   the nitrogen liquefier further comprises a subcooler.

In another aspect of the invention, a method for the production ofliquefied natural gas (“LNG”) and liquid nitrogen (“LIN”) is provided.In this embodiment, the method can include the steps of: a) providing anitrogen refrigeration cycle, wherein the nitrogen refrigeration cycleis configured to provide refrigeration within a heat exchanger, whereina portion of the nitrogen within the nitrogen refrigeration cycle iswithdrawn and liquefied yielding a liquid nitrogen product, wherein atleast an equal portion of gaseous nitrogen is introduced to the nitrogenrefrigeration cycle as is withdrawn; b) cooling and liquefying a firstnatural gas stream in a heat exchanger by heat exchange with nitrogenfrom the nitrogen refrigeration cycle to produce an LNG stream, whereinthe LNG stream is liquefied at a first pressure; c) expanding a secondnatural gas stream to a second pressure to produce an expanded naturalgas stream; and d) warming the expanded natural gas stream in the heatexchanger to produce a warmed natural gas stream, wherein step d)provides a portion of the refrigeration used to cool and liquefy thefirst natural gas stream.

In optional embodiments of the method for the production of LNG and LIN:

-   -   the first natural gas stream comes from a first natural gas        source, and the second natural gas stream comes from a second        natural gas source, wherein the first and second natural gas        sources are different sources;    -   the liquid nitrogen product has a LIN refrigeration requirement,        wherein the LIN refrigeration requirement is supplied by a        combination of refrigeration from the nitrogen refrigeration        cycle and step d); and/or    -   the first natural gas liquefied in step b) is derived from the        expanded natural gas stream, wherein the first pressure and the        second pressure are about the same.

In another aspect of the invention, a method for the production ofliquefied natural gas (“LNG”) and liquid nitrogen (“LIN”) is provided.In this embodiment, the method can include the steps of: a) providing anitrogen refrigeration cycle, wherein the nitrogen refrigeration cyclecomprises a recycle compressor, a turbine, a booster and a plurality ofcoolers, wherein the turbine and booster are configured such that theturbine is configured to power the booster, wherein a portion of thenitrogen within the nitrogen refrigeration cycle is withdrawn andliquefied yielding a liquid nitrogen product, wherein at least an equalportion of gaseous nitrogen is introduced to the nitrogen refrigerationcycle as is withdrawn; b) providing a high pressure natural gas stream;c) splitting the high pressure natural gas stream into a first portionand a second portion; d) cooling and liquefying the first portion of thehigh pressure natural gas stream to produce an LNG stream; e) providinga first portion of refrigeration via the nitrogen refrigeration cycle;f) providing a second portion of refrigeration by expanding the secondportion of the high pressure natural gas; and g) using the first portionof refrigeration and the second portion of refrigeration to achieve thecooling and liquefaction of the first portion of the high pressurenatural gas stream in step d).

In another aspect of the invention, a method for the integration of anitrogen liquefier and natural gas liquefier for the production ofliquefied natural gas (“LNG”) and liquid nitrogen (“LIN”) is provided.In this embodiment, the method can include the steps of: a) providing anitrogen liquefier having a first nitrogen refrigeration cycle, whereinthe nitrogen liquefier comprises a turbine, a booster and a plurality ofcoolers, wherein the first nitrogen refrigeration cycle is configured toprovide refrigeration within a first heat exchanger; b) providing asecond nitrogen refrigeration cycle, wherein the second nitrogenrefrigeration cycle comprises a second turbine, a second booster and aplurality of second coolers, wherein the second nitrogen refrigerationcycle is configured to provide refrigeration within a second heatexchanger; c) purifying a first natural gas stream in a firstpurification unit to remove a first set of impurities to produce apurified first natural gas stream; d) cooling and liquefying the firstnatural gas stream in the second heat exchanger using the refrigerationfrom the nitrogen refrigeration cycle to produce an LNG stream, whereinthe first natural gas stream has an LNG refrigeration requirement,wherein the LNG stream is liquefied at a first pressure P_(H); e)purifying a second natural gas stream in a second purification unit toremove a second set of impurities to produce a purified second naturalgas stream; f) partially cooling the second natural gas stream in thesecond heat exchanger; g) withdrawing the partially cooled natural gasstream from an intermediate section of the second heat exchanger; h)expanding the partially cooled natural gas stream to a medium pressureP_(M) in a natural gas expansion turbine to form a cold natural gasstream, wherein the medium pressure P_(M) is at a pressure lower thanthe first pressure P_(H); and i) warming the cold natural gas stream inthe second heat exchanger by heat exchange against the first natural gasstream to produce a warm natural gas stream at the warm end of thesecond heat exchanger, wherein the natural gas expansion turbine drivesa first booster, wherein the LNG refrigeration requirement is suppliedby a combination of refrigeration from the second nitrogen refrigerationcycle and step i), wherein a portion of the liquid nitrogen within thefirst nitrogen refrigeration cycle is withdrawn as product liquidnitrogen, wherein at least an equal portion of gaseous nitrogen isintroduced to the first nitrogen refrigeration cycle as is withdrawn asproduct liquid nitrogen, and wherein the first nitrogen refrigerationcycle and the second nitrogen refrigeration cycle share a commonnitrogen recycle compressor.

In optional embodiments of the method integration of a nitrogenliquefier and natural gas liquefier:

-   -   the first booster is configured to compress the second natural        gas stream or a stream derived from the second natural gas        stream;    -   the first booster is configured to compress a stream selected        from the group consisting of the first natural gas stream, the        purified first natural gas stream, the second natural gas        stream, the purified second natural gas stream, the partially        cooled natural gas stream, the warm natural gas stream, and a        nitrogen fluid within the nitrogen refrigeration cycle; the        first set of impurities has a freezing point at or above the        liquefaction temperature of methane at the first pressure P_(H);    -   the second set of impurities comprises water;    -   the first nitrogen refrigeration cycle further comprises a        nitrogen feed compressor;    -   the first nitrogen refrigeration cycle is a closed refrigeration        cycle;    -   the first natural gas stream and the second natural gas stream        come from the same natural gas source;    -   the natural gas source is a natural gas pipeline having a        pressure between 15 and 100 bara;    -   the first natural gas stream comes from a first natural gas        source, and the second natural gas stream comes from a second        natural gas source, wherein the first and second natural gas        sources are different sources;    -   the first natural gas source comprises a natural gas pipeline;    -   the natural gas pipeline has a pressure between 15 and 100 bara;    -   the first purification unit and the second purification unit are        the same unit;    -   the first purification unit and the second purification unit are        separate units, wherein the first purification unit is        configured to remove at least water and carbon dioxide, and        wherein the second purification unit is configured to remove at        least water; and/or    -   the nitrogen liquefier further comprises a subcooler.

In another aspect of the invention, a method for the integration of afirst liquefier and a second liquefier for the production of firstliquefied gas and a second liquefied gas is provided. In thisembodiment, the method can include the steps of: a) providing a firstliquefier having a first refrigeration cycle, wherein the firstliquefier comprises a recycle compressor, a first heat exchanger, and aturbine booster; b) providing a second refrigeration cycle, wherein thesecond refrigeration cycle is configured to provide refrigeration withina second heat exchanger, c) cooling and liquefying a first gas stream inthe second heat exchanger by heat exchange with the second refrigerationcycle to produce a liquefied first gas stream, wherein the liquefiedfirst gas stream is at a first pressure; d) expanding a second gasstream to a second pressure to produce an expanded second gas stream;and e) warming the expanded second gas stream in the second heatexchanger to produce a warmed gas stream, wherein a portion of a firstrefrigeration gas within the first refrigeration cycle is withdrawn andliquefied yielding a liquid first refrigeration gas product, wherein atleast an equal portion of gaseous first refrigeration gas is introducedto the first refrigeration cycle as is withdrawn as liquid firstrefrigeration gas product, wherein step e), in addition to therefrigeration provided by the second refrigeration cycle, provides therefrigeration used to cool and liquefy the first gas stream, and whereinthe first refrigeration cycle and the second refrigeration cycle share acommon recycle compressor.

In optional embodiments of the method for the integration of a firstliquefier and a second liquefier:

-   -   the first refrigeration cycle is selected from the group        consisting of a nitrogen refrigeration cycle and a hydrogen        refrigeration cycle;    -   the first gas stream liquefied in step c) is derived from the        expanded second gas stream, wherein the first pressure and the        second pressure are about the same;    -   the second refrigeration cycle is selected from the group        consisting of a nitrogen refrigeration cycle and a hydrogen        refrigeration cycle;    -   the first gas stream cooled and liquefied in step c) comprises        natural gas;    -   the second gas stream expanded in step d) comprises natural gas;        and/or    -   the liquid first refrigeration gas product is liquid nitrogen.

In another aspect of the invention, a method for the integration of afirst liquefier and a second liquefier for the production of a firstliquefied gas and a second liquefied gas is provided. In thisembodiment, the method can include the steps of: a) providing a firstliquefier having a first refrigeration cycle using a first refrigerant,wherein the first refrigeration cycle is configured to providerefrigeration within a first heat exchanger; b) providing a secondliquefier having a second refrigeration cycle using a secondrefrigerant, wherein the second refrigeration cycle is configured toprovide refrigeration within a second heat exchanger; c) cooling a firstgas stream in the first heat exchanger by heat exchange with the firstrefrigeration cycle to produce a cooled first gas stream; d) cooling asecond gas stream in the second heat exchanger by heat exchange with thesecond refrigeration cycle to produce a cooled second gas stream; e)expanding a third gas stream to produce an expanded third gas stream;and f) warming the expanded third gas stream in a heat exchangerselected from the group consisting of the first heat exchanger, thesecond heat exchanger, and combinations thereof, to produce a warmed gasstream, wherein step f), in addition to the refrigeration provided bythe second refrigeration cycle, provides the refrigeration used to coolthe second gas stream, wherein step f), in addition to the refrigerationprovided by the first refrigeration cycle, provides the refrigerationused to cool the first gas stream, and wherein the first refrigerationcycle and the second refrigeration cycle share a common recyclecompressor.

In optional embodiments of the method for the integration of a firstliquefier and a second liquefier:

-   -   the first and second refrigeration cycles are nitrogen        refrigeration cycles;    -   the first refrigerant and the second refrigerant have the same        composition;    -   the first gas stream is selected from the group consisting of        natural gas, ethane, ethylene, acetylene, other C3-C6 alkanes,        alkenes, and alkynes, and nitrogen, and wherein the first gas        stream is liquefied during cooling step c);    -   the first gas stream is selected from the group consisting of        hydrogen and helium, wherein the first gas stream is not        liquefied during cooling step c);    -   the third gas stream expanded in step e) comprises natural gas;    -   a portion of the first refrigerant within the first        refrigeration cycle is withdrawn and liquefied yielding a liquid        first refrigerant product, wherein at least an equal portion of        gaseous first refrigerant is introduced to the first        refrigeration cycle as is withdrawn as liquid first refrigerant;        and/or    -   a portion of the second refrigerant within the second        refrigeration cycle is withdrawn and liquefied yielding a liquid        second refrigerant, wherein at least an equal portion of the        second refrigerant is introduced to the second refrigeration        cycle as is withdrawn as liquid second refrigerant.

In another aspect of the invention, a method for the integration of anitrogen liquefier and letdown of natural gas for the production liquidnitrogen (“LIN”) is provided. In this embodiment, the method can includethe steps of: a) providing a nitrogen liquefier having a nitrogenrefrigeration cycle, wherein the nitrogen liquefier comprises a nitrogenrecycle compressor, a heat exchanger, and a first turbine booster; b)introducing a nitrogen gas stream to the nitrogen liquefier underconditions effective for liquefying the nitrogen to produce a liquidnitrogen product; c) withdrawing a natural gas stream from a sourceoperating at a first pressure P_(H); d) purifying the natural gas streamin a purification unit to produce a purified natural gas; e) partiallycooling the purified natural gas in the heat exchanger; f) withdrawingthe partially cooled natural gas from an intermediate section of theheat exchanger; g) expanding the partially cooled natural gas to amedium pressure P_(M) in a natural gas expansion turbine to form a coldnatural gas stream, wherein the medium pressure P_(M) is at a pressurelower than the first pressure P_(H); and h) warming the cold natural gasstream in the heat exchanger by heat exchange against nitrogen from thenitrogen refrigeration cycle to produce a warm natural gas stream from awarm end of the heat exchanger, wherein step h) provides additionalrefrigeration to the nitrogen liquefier such that additional liquidnitrogen can be produced as compared to a method having an absence ofsteps c)-i), wherein the natural gas expansion turbine drives a firstgas booster.

In optional embodiments of the method for the integration of a nitrogenliquefier and letdown of a natural gas stream:

-   -   the first gas booster is configured to compress the natural gas        stream or a stream derived therefrom;    -   the first gas booster is configured to compress a stream        selected from the group consisting of the natural gas stream,        the purified natural gas stream, the partially cooled natural        gas stream, the warm natural gas stream, and a nitrogen fluid        within the nitrogen refrigeration cycle;    -   the purification unit is configured to remove at least water        from the natural gas stream;    -   the nitrogen refrigeration cycle further comprises a nitrogen        feed compressor;    -   the nitrogen liquefier further comprises a subcooler;    -   the source of the natural gas comprises a natural gas pipeline;        and/or    -   the natural gas pipeline has a pressure between 15 and 100 bara.

In another aspect of the invention, a method for integration of anitrogen liquefier and letdown of natural gas for the production ofliquid nitrogen (“LIN”) is provided. In this embodiment, the method caninclude the steps of: a) providing a nitrogen liquefier having anitrogen refrigeration cycle, wherein the nitrogen liquefier comprises anitrogen recycle compressor, a heat exchanger, and at least one turbinebooster; b) introducing a nitrogen gas stream to the nitrogen liquefierunder conditions effective for liquefying the nitrogen to produce aliquid nitrogen product; c) recovering a natural gas stream from a highpressure source, wherein the natural gas stream is at a first pressure;d) expanding the natural gas stream to a second pressure to produce anexpanded natural gas stream, wherein the second pressure is a pressurethat is lower than the first pressure; and e) warming the expandednatural gas stream in the heat exchanger to produce a warmed natural gasstream, wherein step e) provides additional refrigeration to thenitrogen liquefier such that additional liquid nitrogen can be producedas compared to a method having an absence of steps d) through e).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of theinvention and are therefore not to be considered limiting of theinvention's scope as it can admit to other equally effectiveembodiments.

FIG. 1 provides an embodiment of the prior art.

FIG. 2 provides an embodiment of the present invention.

FIG. 3 provides an embodiment of the present invention with both LIN andLNG production.

FIG. 4 provides another embodiment of the present invention with bothLIN and LNG production.

FIG. 5 provides an embodiment of the present invention with LIN andmedium pressure natural gas production.

DETAILED DESCRIPTION

While the invention will be described in connection with severalembodiments, it will be understood that it is not intended to limit theinvention to those embodiments. On the contrary, it is intended to coverall the alternatives, modifications and equivalence as may be includedwithin the spirit and scope of the invention defined by the appendedclaims.

In one embodiment, the method can include integrating a natural gasletdown system with a nitrogen refrigeration cycle. In one embodiment,the nitrogen refrigeration cycle is a closed loop refrigeration cycle.In this embodiment, the natural gas letdown essentially provides “free”refrigeration energy since the natural gas would have been alternativelyletdown across a valve (i.e., the resulting drop in temperature of thenatural gas would have been absorbed by the surroundings and would nothave been recovered in any meaningful way). With the addition of anatural gas turbine booster, LNG can be co-produced with a significantpower savings, while also potentially reducing the size of the nitrogenrefrigeration cycle. In another embodiment, a purification unit,storage, loading and utility systems may also be included.

Referring to FIG. 2, a process flow diagram of an embodiment of thecurrent invention is shown. In FIG. 2, high pressure natural gas 2 ispreferably split into two portions, with one portion being liquefied andthe other portion providing a portion of the refrigeration used to cooland liquefy the natural gas. First portion of the natural gas stream 102is purified in first purification unit 130, wherein acid gases, waterand mercury are preferably removed. Preferably, any impurity within thenatural gas that would solidify prior to the natural gas liquefying ordamage the downstream equipments is removed in first purification unit130. The resulting purified first portion of the natural gas stream 104is then withdrawn from first purification unit 130 and introduced toheat exchanger 40 for liquefaction therein. In embodiments in which thenatural gas feed contains heavy hydrocarbons, it is preferable towithdraw purified first portion of the natural gas 104 from anintermediate section of heat exchanger 40 and separate the heavyhydrocarbons 8 using gas liquid separator 5. Alternatively, thegas-liquid separator may be replaced by a distillation column or otherseparation devices known in the art. Instead of collecting heavyhydrocarbons 8 separately as shown in FIG. 1, heavy hydrocarbons 8 maybe expanded and then warmed in heat exchanger 40. The resulting warmedstream can be combined with other natural gas streams (e.g., coldnatural gas stream 144 and first portion of the LNG 146) within heatexchanger 40. This advantageously captures some of the cold energy fromheavy hydrocarbons 8, and if warm natural gas stream 108 is subsequentlyused for fuel, it also provides additional energy for that purpose.

Vaporized natural gas from gas liquid separator 5 is reintroduced toheat exchanger 40, wherein it subsequently liquefies to produce LNG 6.In one embodiment, first portion of the LNG 146 can be removed from LNG6, expanded in second valve V2, and then warmed in heat exchanger 40,thereby providing additional refrigeration, to produce warm natural gasstream 108. The remaining portion can then be expanded across thirdvalve V3, thereby producing low pressure LNG 148.

Refrigeration for the system is provided by two sources. The firstrefrigeration source can be via a conventional nitrogen refrigerationcycle 50. Nitrogen gas is compressed in nitrogen recycle compressor 10,cooled in cooler 11, compressed further in booster of first turbinebooster 20, cooled in cooler 21, then further compressed in booster ofsecond turbine booster 25 before being cooled again in cooler 26. Theresulting compressed nitrogen is then cooled in heat exchanger 40,wherein a first portion is removed and expanded in turbine of secondturbine booster 25 and the remaining portion is removed and expanded inturbine of first turbine booster 20. The resulting expanded nitrogenstreams are then introduced to heat exchanger 40, where they are warmedvia indirect heat exchange against the natural gas and other nitrogenstreams.

The second refrigeration source is provided by using the excess pressuredifferential of the high pressure natural gas. In this embodiment,second portion of the natural gas stream 106 is split from high pressurenatural gas 2, and then purified in second purification unit 131 of atleast water and potentially mercury to produce purified second portionof the natural gas 132. While the embodiment shown in FIG. 2 includestwo separate purification units, it is possible to use a singlepurification unit to fully purify the entire natural gas stream prior tosplitting the natural gas into two streams. However, it is preferable tosplit the streams prior to purification since the natural gas used toprovide refrigeration (i.e., the portion not liquefied), does not needto have carbon dioxide removed, since the natural gas turbine outletstream 144 is at a sufficiently warm temperature such that carbondioxide will not freeze within this stream. In another embodiment, units130 and 131 may be combined into a single unit, and the moisture freestream (e.g., 132) is removed at an intermediate location of the vesseland the moisture and CO₂ free stream (e.g., 104) is removed from the endof the vessel opposite the feed location.

Purified second portion of the natural gas 132 is then compressed inbooster of natural gas turbine booster 120, cooled in cooler 140 toproduce compressed natural gas stream 142. Compressed natural gas stream142 can then be partially cooled in heat exchanger 40, before beingexpanded in turbine of natural gas turbine booster 120 to form coldnatural gas stream 144. Alternatively, in an embodiment not shown,natural gas stream 142 can be sent, prior to cooling, directly tonatural gas turbine 120 for expansion. This can help limit thetemperature of 144 to avoid heavy hydrocarbon condensation and potentialsolidification. Cold natural gas stream 144 is then reintroduced to heatexchanger 40, wherein it is warmed via indirect heat exchange andcollected as warm natural gas stream 108 from the warm end of the heatexchanger. In one embodiment, cold natural gas stream 144 can becombined with heavy hydrocarbons 8 and optionally first portion of theLNG 146 within the heat exchanger, or the different streams can warmindividually within the heat exchanger and be combined following theirwarming.

The booster of natural gas turbine booster 120 can be located at manydifferent locations depending on the natural gas source and returnpressures. For example, it may be located at 1) the NG stream to beexpanded (FIG. 2) if the feed pressure and/or return pressure are low,2) the total natural gas feed flow before splitting the flow to beexpanded and flow to be liquefied (FIG. 3), or 3) on the discharge ofthe turbine at the warm end of the exchanger (e.g., stream 108) in thecase of high natural gas feed pressure and high natural return pressure(not shown), or 4) on the natural gas stream to be liquefied (e.g.,stream 104) if the feed pressure is low (not shown). Alternatively theturbine may be used to drive an electrical generator or dissipated byoil brake (not shown).

A comparison of the embodiment shown in FIGS. 1 and 2 can be found inTable I below:

TABLE I Comparison of Energy Requirements for FIG. 1 and FIG. 2 Base(Typical LNG LNG production by production by N2 NG letdown and N2 cycle)FIG. 1 cycle. FIG. 2 NG supply 32 bara 342 1019 (mtd) NG to letdown 5.60 677 bara (mtd) LNG Production 342 342 (mtd) N2 cycle power input 71554158 (kW) LNG Specific power 502 292 (kWh/mt) Power Reduction (%) — 42%LIN production — — (mtd) LIN Specific Power — — (kWh/mt)

In the setup shown of FIG. 2, the power required to produce 342 mtd ofLNG is reduced to approximately 4158 kW, meaning the specific power ofthis setup is approximately 292 kWh/mt. As such, this represents adecrease of approximately 42% in power requirements.

Regarding FIG. 3, a process flow diagram of an embodiment for theco-production of liquid nitrogen and LNG using a nitrogen refrigerationcycle in combination with natural gas letdown. In FIG. 3, natural gascan be acquired from a natural gas source, compressed in natural gasbooster 101 to produce high pressure natural gas 2. High pressurenatural gas 2 is preferably split into two portions, with one portionbeing liquefied and the other portion providing a portion of therefrigeration used to cool and liquefy the natural gas. First portion ofthe natural gas stream 102 is purified in first purification unit 130,wherein acid gases, water and mercury are preferably removed.Preferably, any impurity within the natural gas that would damage orsolidify prior to the natural gas liquefying is removed in firstpurification unit 130. The resulting purified first portion of thenatural gas stream 104 is then withdrawn from first purification unit130 and introduced to heat exchanger 40 for liquefaction therein. Inembodiments in which the natural gas feed contains heavy hydrocarbons,it is preferable to withdraw purified first portion of the natural gas104 from an intermediate section of heat exchanger 40 and separate theheavy hydrocarbons 8 using gas liquid separator 5. Alternatively, thegas-liquid separator may be replaced by a distillation column or otherseparation devices known in the art. Instead of collecting heavyhydrocarbons 8 separately as shown in FIG. 1, heavy hydrocarbons 8 maybe expanded and then warmed in heat exchanger 40. The resulting warmedstream can be combined with cold natural gas stream 144 within heatexchanger 40. This advantageously captures some of the cold energy fromheavy hydrocarbons 8, and if warm natural gas stream 108 is subsequentlyused for fuel, it also provides additional energy for that purpose.

Vaporized natural gas from gas liquid separator 5 is reintroduced toheat exchanger 40, wherein it subsequently liquefies to produce LNG 6.While not shown specifically in FIG. 3, as in FIG. 2, in one embodiment,first portion of the LNG 146 can be removed from LNG 6, expanded insecond valve V2, and then warmed in heat exchanger 40, thereby providingadditional refrigeration, to produce warm natural gas stream 108. Theremaining portion can then be expanded across third valve V3, therebyproducing second portion of the LNG 148. In the embodiment shown in FIG.3, all of LNG 6 is expanded in valve V3 and used as product.

Refrigeration for the system is provided by two sources. The firstrefrigeration source can be via a conventional nitrogen refrigerationcycle 50. Nitrogen gas is compressed in nitrogen recycle compressor 10,cooled in cooler 11, compressed further in booster of first turbinebooster 20, cooled in cooler 21, then further compressed in booster ofsecond turbine booster 25 before being cooled again in cooler 26. Theresulting compressed nitrogen is then cooled in heat exchanger 40,wherein a first portion is removed and expanded in turbine of secondturbine booster 25, a second portion is removed and expanded in turbineof first turbine booster 20. The resulting expanded nitrogen streams arethen introduced to heat exchanger 40, where they are warmed via indirectheat exchange against the natural gas and other nitrogen streams.

The second refrigeration source is provided by using the excess pressuredifferential of the high pressure natural gas. In this embodiment,second portion of the natural gas stream 106 is split from high pressurenatural gas 2, and then purified in second purification unit 131 of atleast water and preferably mercury to produce purified second portion ofthe natural gas 132. While the embodiment shown in FIG. 3 includes twoseparate purification units, it is possible to use a single purificationunit to fully purify the entire natural gas stream prior to splittingthe natural gas into two streams. However, it is preferable to split thestreams prior to purification since the natural gas used to providerefrigeration (i.e., the portion not liquefied), does not need to havecarbon dioxide removed, since the natural gas turbine outlet stream 144is at a sufficiently warm temperature such that carbon dioxide will notfreeze within this stream. In another embodiment, units 130 and 131 maybe combined into a single unit, and the moisture free stream (e.g., 132)is removed at an intermediate location of the vessel and the moistureand CO₂ free stream (e.g., 104) is removed from the end of the vesselopposite the feed location.

Purified second portion of the natural gas 132 can then be partiallycooled in heat exchanger 40, before being expanded in turbine 121 ofnatural gas turbine booster 120 to form cold natural gas stream 144.Alternatively, in an embodiment not shown, purified second portion ofthe natural gas stream 132 can be sent, prior to cooling, directly tonatural gas turbine 121 for expansion. This can help limit thetemperature of 144 to avoid heavy hydrocarbon condensation and potentialsolidification. Cold natural gas stream 144 is then reintroduced to heatexchanger 40, wherein it is warmed via indirect heat exchange andcollected as warm natural gas stream 108 from the warm end of the heatexchanger. In one embodiment, cold natural gas stream 144 can becombined with heavy hydrocarbons 8 within the heat exchanger, or thedifferent streams can warm individually within the heat exchanger and becombined following their warming.

The booster 101 of natural gas turbine booster 120 can be located atmany different locations depending on the natural gas source and returnpressures. For example, it may be located at 1) the NG stream to beexpanded (FIG. 2) if the feed pressure and/or return pressure are low,2) the total natural gas feed flow before splitting the flow to beexpanded and flow to be liquefied (FIG. 3), or 3) on the discharge ofthe turbine at the warm end of the exchanger (e.g., stream 108) in thecase of high natural gas feed pressure and high natural return pressure(not shown), or 4) on stream to be liquefied (e.g., stream 104) if thefeed pressure is low (not shown). Alternatively the turbine may be usedto drive an electrical generator or dissipated by oil brake (not shown).

The primary difference between the embodiment of FIG. 2 and theembodiment of FIG. 3 is that in FIG. 3, low pressure gaseous nitrogen isintroduced as feed into the nitrogen refrigeration cycle and LIN iscoproduced with LNG. In one particular embodiment, gaseous nitrogen(“GAN”) is introduced into, and compressed by, nitrogen compressor 15before being cooled in cooler 16 and then added to the refrigerationcycle. Those of ordinary skill in the art will recognize that thenitrogen compressor 15 can be optional, since its use can be dependenton the pressure of the GAN feed stream. In another embodiment, a thirdportion of the cooled nitrogen is removed from the heat exchanger 40,subcooled in nitrogen subcooler 45, and expanded across valve V4 beforebeing introduced to nitrogen gas liquid separator 55. Nitrogen vapor 57is withdrawn from the top of nitrogen gas liquid separator 55 and thenwarmed in heat exchanger 40, wherein it is then recompressed by nitrogencompressor 15 before again rejoining the refrigeration cycle. Liquidnitrogen is withdrawn from the bottom of nitrogen gas liquid separator55 and preferably one portion 51 is sent to be vaporized in subcooler45, while the other portion 52 is sent to a liquid nitrogen storage tank(not shown).

As such, FIG. 3 provides for an embodiment in combining LIN+LNG+naturalgas letdown. As before, the nitrogen refrigeration cycle includes arecycle compressor, and at least one turbine booster. However, becauseit produces LIN (e.g., removes nitrogen molecules from the loop), italso includes a step of adding gaseous nitrogen feed to the system. Inthe embodiment shown in FIG. 3, the gaseous nitrogen makeup is at lowpressure, and therefore it also includes a nitrogen feed compressor, aswell as a subcooler to provide liquid nitrogen product. As in otherembodiments, the natural gas supply is split between a flow to beliquefied and a flow to be expanded back to low pressure. As notedpreviously, the natural gas booster 101 may be located at variouslocations depending on the flow ratio and pressure of the natural gasfeed and letdown pressures used.

Regarding FIG. 4, a process flow diagram of an embodiment having apartial integration of a nitrogen liquefier with a natural gas liquefieris shown. In FIG. 4, natural gas can be acquired from a natural gassource, compressed in natural gas booster 101 to produce high pressurenatural gas 2. High pressure natural gas 2 is preferably split into twoportions, with one portion being liquefied and the other portionproviding a portion of the refrigeration used to liquefy the naturalgas. First portion of the natural gas stream 102 is purified in firstpurification unit 130, wherein acid gases, water and mercury arepreferably removed. Preferably, any impurity within the natural gas thatwould damage equipment or solidify prior to the natural gas liquefyingis removed in first purification unit 130. The resulting purified firstportion of the natural gas stream 104 is then withdrawn from firstpurification unit 130 and introduced to heat exchanger 440 forliquefaction therein. In embodiments in which the natural gas feedcontains heavy hydrocarbons, it is preferable to withdraw purified firstportion of the natural gas 104 from an intermediate section of heatexchanger 440 or before entering exchanger 440 and separate the heavyhydrocarbons 8 using gas liquid separator 5 or distillation column. Inone embodiment, heavy hydrocarbons 8 may be expanded and then warmed inheat exchanger 440. The resulting warmed stream can be combined withother natural gas streams (e.g., cold natural gas stream 144) withinheat exchanger 440. This advantageously captures some of the cold energyfrom heavy hydrocarbons 8, and if warm natural gas stream 108 issubsequently used for fuel, it also provides additional energy for thatpurpose. Vaporized natural gas from gas liquid separator 5 isreintroduced to heat exchanger 440, wherein it subsequently liquefies toproduce LNG 6.

Refrigeration for the system can be provided by three sources, a firstnitrogen refrigeration cycle 50, a second nitrogen refrigeration cycle450, and by expansion of high pressure natural gas. In first nitrogenrefrigeration cycle 50, nitrogen gas coming from first nitrogenrefrigeration cycle 50 and second nitrogen refrigeration cycle 450 iscompressed in shared nitrogen recycle compressor 410, and cooled incooler 411. The resulting compressed nitrogen is then split into twostreams, with a first portion going to first nitrogen refrigerationcycle 50 and the second portion going to second nitrogen refrigerationcycle 450.

With respect to first nitrogen refrigeration cycle 50, the nitrogen canbe compressed further in booster of first turbine booster 20, cooled incooler 21, further compressed in booster of second turbine booster 25before being cooled again in cooler 26. The resulting compressednitrogen is then cooled in heat exchanger 40, wherein a first portion isremoved and expanded in turbine of second turbine booster 25, a secondportion is removed and expanded in turbine of first turbine booster 20.The resulting expanded nitrogen streams are then introduced to heatexchanger 40, where they are warmed via indirect heat exchange againstthe natural gas and other nitrogen streams, and then sent back to sharednitrogen recycle compressor 410.

As in FIG. 3, the embodiment of FIG. 4 also includes low pressuregaseous nitrogen introduced as feed and LIN is coproduced. Gaseousnitrogen (GAN) is introduced into, and compressed by, nitrogencompressor 15 before being cooled in cooler 16 and then added to therefrigeration cycle. Those of ordinary skill in the art will recognizethat the nitrogen compressor 15 can be optional, since its use can bedependent on the pressure of the GAN feed stream. Additionally, theremaining portion of the compressed nitrogen is removed from the heatexchanger 40, subcooled in nitrogen subcooler 45, and expanded acrossvalve V4 before being introduced to nitrogen gas liquid separator 55.Nitrogen vapor 57 is withdrawn from the top of nitrogen gas liquidseparator 55 and then warmed in heat exchanger 40, wherein it is thenrecompressed by nitrogen compressor 15 before again rejoining therefrigeration cycle. Liquid nitrogen is withdrawn from the bottom ofnitrogen gas liquid separator 55 then split into first portion 51 whichis vaporized in subcooler 45 to provide heat exchange for the LINsubcooling and second portion 52 as LIN production preferably sent to astorage tank (not shown).

The second refrigeration source can be second nitrogen refrigerationcycle 450, which is comprised of shared nitrogen recycle compressor 410,shared cooler 411, and non-shared equipment such as third turbinebooster 420, cooler 421, fourth turbine booster 425, and cooler 426.

The third source of refrigeration is provided by using available excesspressure differential of high pressure natural gas. In this embodiment,second portion of the natural gas stream 106 is split from high pressurenatural gas 2, and then purified in second purification unit 131 of atleast water and preferably mercury to produce purified second portion ofthe natural gas 132. While the embodiment shown in FIG. 4 includes twoseparate purification units, it is possible to use a single purificationunit to fully purify the entire natural gas stream prior to splittingthe natural gas into two streams. However, it is preferable to split thestreams prior to purification since the natural gas used to providerefrigeration (i.e., the portion not liquefied), does not need to havecarbon dioxide removed, since the natural gas turbine outlet stream 144is at a sufficiently warm temperature such that carbon dioxide will notfreeze within this stream. Alternatively, units 130 and 131 may becombined into a single unit such that the moisture free stream 132 isremoved at an intermediate location of the vessel and the moisture andCO₂ free stream 104 is removed from the end of the vessel opposite thefeed 2 location.

Purified second portion of the natural gas 132 may be partially cooledin heat exchanger 440, before being expanded in natural gas turbine 121to form cold natural gas stream 144. Alternatively, stream 132 can besent, prior to cooling in the heat exchanger, to turbine 121 forexpansion to limit the temperature of 144 due to CO₂ freezing or heavyhydrocarbon condensation. Cold natural gas stream 144 is thenreintroduced to heat exchanger 440, wherein it is warmed via indirectheat exchange and collected as warm natural gas stream 108 from the warmend of the heat exchanger. In one embodiment, cold natural gas stream144 can be combined with heavy hydrocarbons 8 within the heat exchanger,or the two streams can warm individually within the heat exchanger andbe combined following their warming.

The booster 101 of natural gas turbine booster 120 can be located atmany different locations depending on the natural gas source and returnpressures. For example, it may be located at 1) the NG stream to beexpanded (FIG. 2) if the feed pressure and/or return pressure are low,2) the total natural gas feed flow before splitting the flow to beexpanded and flow to be liquefied (FIG. 3), or 3) on the discharge ofthe turbine at the warm end of the exchanger (e.g., 108) in the case ofhigh natural gas feed pressure and high natural return pressure (notshown), or 4) on stream to be liquefied (e.g., 104) if the feed pressureis low (not shown). Alternatively the turbine may be used to drive anelectrical generator or dissipated by oil brake (not shown).

As noted above, the embodiment of FIG. 4 preferably includes astand-alone nitrogen liquefier 350, that shares a common nitrogenrecycle compressor (e.g., 410), with the second nitrogen refrigerationcycle 450. As such, such an embodiment can advantageously produce LINand LNG at locations that have both a nitrogen liquefaction unit andaccess to natural gas.

The embodiment of FIG. 4 has a 12% efficiency improvement compared tothe embodiment shown in FIG. 3, primarily due to the additional turbineboosters which can be positioned at temperatures in the cycle toindependently optimize the LNG and LIN trains.

Additionally, the shared recycle compressor 410 provides a lower capitalcost compared to an independent nitrogen liquefier plus independent LNGplant, since the embodiment effectively eliminates one recyclecompressor, which typically is the largest capital cost equipment of thesystem. In addition, there is a small efficiency improvement due to asingle, large machine compared to two, small machines. Similarly asindicated before, the location of the booster for the natural gasletdown can vary with natural gas source and letdown pressure.

A comparison of the embodiments shown in FIGS. 1-4 can be found in TableII below.

TABLE II Comparison Data for FIGS. 1-4 LNG + LIN Base (Typical LNG LNGproduction by production by NG LNG + LIN production production by N2 NGletdown and N2 letdown and N2 by NG letdown and N2 cycle) FIG. 1 cycle.FIG. 2 cycle (FIG. 3) cycle (FIG. 4) NG supply 32 bara 342 1019 10191019 (mtd) NG to letdown 5.6 0 677 677 677 bara (mtd) LNG Production 342342 342 342 (mtd) N2 cycle power input 7155 4158 10555 9974 (kW) LNGSpecific power 502 292 353 313 (kWh/mt) Power Reduction (%) — 42% 30%38% LIN production — — 301 301 (mtd) LIN Specific Power — — 440 440(kWh/mt)

In an optional embodiment, fourth gas stream 351 can be cooled and/orliquefied within heat exchanger 40 to produce cooled/liquefied fourthgas stream 352. In one embodiment, fourth gas stream 351 is selectedfrom the group consisting of natural gas; ethane; ethylene; acetylene;C₃-C₆ alkanes, alkenes and alkynes; nitrogen; hydrogen; and helium. Inembodiments in which gas stream 351 is hydrogen or helium, gas stream352 is preferably not liquefied. Otherwise, cooled stream 352 ispreferably liquefied. Advantageously, this optional embodiment allowsfor three separate gases to be liquefied (e.g., streams 52, 352 and 6).

The embodiments shown in FIG. 3 and FIG. 4 are preferably located near,on, or have access to an industrial site with a large constant letdownflow of natural gas (e.g., a cogen unit, or steam methane reformerfacility), as well as a source of nitrogen (e.g., near an air separationunit “ASU” or nitrogen pipeline). Nitrogen is often available near anASU as they are commonly designed for O₂ production. Nitrogen may beextracted with a small cost to the ASUs precooling system.

The embodiment shown in FIG. 4 includes a specific embodiment ofproducing LNG and LIN, however, the invention is not to be so limited.Instead, an embodiment of the invention can include liquefaction of afirst gas and a second gas, through the use of two refrigeration cycles,in which the two refrigeration cycles share a common recycle compressor.In a preferred embodiment, the refrigeration cycles are nitrogenrefrigeration cycles. In one embodiment, the two liquefiers could eachproduce either LIN or LNG or liquid hydrogen or liquid helium or anytype of other industrial gases. In another embodiment, either or both ofthe liquefiers may have an expansion device configured to expand ahigher pressure gas source.

Embodiments of the invention can have wide applications in the industry.For example, an embodiment of the invention may include identifying anunderutilized liquefaction system, and then adding a second liquefiernearby (e.g., an LNG liquefier). The original liquefier can be slightlymodified in order to allow for its previously underutilized recyclecompressor to provide compression for both refrigeration cycles. Thisallows for the new liquefier to produce its liquid in a much moreefficient manner. In another embodiment, the second liquefaction unit ispreferably located nearby a high and low pressure pipeline network(e.g., natural gas pipeline) such that the system is able to use therefrigeration from expansion of the natural gas.

In another embodiment, two new liquefiers can be built to satisfy amarket demand. For example, the first liquefier can be a nitrogenliquefaction unit and the second liquefier can be a natural gasliquefaction unit, both using nitrogen refrigeration cycles. It can beeconomically advantageous that at least one of the liquefiers is astandardized plant (e.g., a modular type design that can be designed andproduced in bulk). In many cases, the capacity which the standardizedplant has been designed for is greater than the capacity needed for thisspecific application. A similar concept could apply to the relocation ofan existing liquefier. Therefore, the second liquefier can be built suchthat its refrigeration cycle uses the same recycle compressor as the onefrom the first liquefier. It is also common that such liquefactionplants are located near an industrial area, therefore benefiting from awide natural gas pipeline network. One or both liquefiers would benefitfrom adding a natural gas expansion refrigeration to each nitrogenrefrigeration cycle, as described herein.

Similarly, if the standardized plant were undersized for a particularapplication (e.g., produce liquid nitrogen), the second liquefactionunit could be designed to make up the difference. In this embodiment,the second liquefaction unit could be configured to create both a liquidnitrogen product, as well as an LNG product.

Operational problems can occur when the natural gas turbine drives anelectric generator without extracting the refrigeration energy ofexpansion. Furthermore, in some instances, the flow rate and pressuresof the natural gas can often fluctuate. This can cause issues withrespect to fluctuations in produced energy, since electrical systems arenot always able to accept the resulting fluctuations of electricity sentto the grid from the generator. Similarly, the resulting fluctuations incold created by the natural gas expansion can yield fluctuations inother utilities.

In certain embodiments of the invention, the above referenced problemscan be mitigated through the use of an LNG and/or LIN storage tank, asthe storage tank provides a buffer for the fluctuations of therefrigeration balance. For example, minor fluctuations in natural gasconditions can be accounted for by adjusting the load of the nitrogenrefrigeration cycle and the quantity of LNG and/or LIN being liquefied.Large or long term fluctuations can be accounted for by stopping theliquefier and compensating by the tank level. In addition, significantshort term fluctuations can be accounted for by adjusting a bypass valveto allow high pressure natural gas to bypass the liquefier and goingstraight to the MP GAN stream (not shown). In another embodiment, themethod can include monitoring various process conditions (e.g.,pressure, flow rate, gas composition, etc. . . . ) of the natural gassource, and/or streams downstream of the natural gas source. Based onthese monitored process conditions, various set points can be adjustedin order to further optimize the system. For example, a set point thatcan be adjusted can include expansion ratio for the various turbines,along with flow rates of various streams throughout. In one embodiment,the set points for the flow rate and inlet pressure to the natural gasturbine can be controlled within an acceptable operating range of theliquefaction equipment by adjustment of the natural gas bypass valveand/or a turbine inlet control valve. In one embodiment, the method caninclude a central process controller that is configured to receive thevarious monitored process conditions and then determine whether aselected set point should be adjusted based on the monitored processconditions. The monitoring devices can communicate with the controllervia all known methods, for example, both wirelessly and via wiredelectrical communication.

FIG. 5 provides for a process flow diagram with liquid nitrogenproduction being supplemented with refrigeration from letdown of naturalgas. The additional energy provided by the natural gas letdown reducesthe power and size of the nitrogen refrigeration cycle for a fixed LINproduction depending on the amount of energy which can be removed fromthe natural gas letdown (i.e., flow and pressure ratio of the NGletdown).

In this type of embodiment, it is preferable that the system beproximate to a nitrogen source (e.g., ASU with available nitrogenproduction, or other small dedicated nitrogen generator, or nitrogenpipeline) as well as a source of pressurized natural gas suitable forletdown. While it is understood that there will be variations in thenatural gas flow and pressure, the liquefier can accommodate some ofthese variations by a corresponding adjustment in LIN production and orpower from the nitrogen refrigeration cycle.

The method shown in FIG. 5 has one natural gas turbine booster for thewarm section of the exchanger and one nitrogen turbine booster for thecold section. However, for improved efficiency and flexibility, anadditional warm turbine booster (as shown in FIG. 2) can be included incertain embodiments of the invention.

With respect to purification, water should be removed and depending onnatural gas composition, pressure and temperature prior to natural gasexpansion, acid gases such as CO₂, and other impurities which freeze atcolder temperatures may be removed from the natural gas as well. Thenatural gas may be cooled before being expanded and can reach atemperature of approximately −60° C. to −100° C. before entering theheat exchanger, is re-warmed and returned to the low pressure header.Since CO₂ will only freeze at lower temperatures, it is not required toremove CO₂ from the stream being expanded.

Since the liquefier is intended to be in industrial facilities withconstant natural gas letdown, nitrogen source, etc, these facilitiesoften have much less impurities in the feed natural gas. For exampleodorization (addition of sulfur containing mercaptans) is not used inthese areas. Therefore, the purification system maybe simplifiedcompared to a similar unit installed at a non-industrial site.

Those of ordinary skill in the art will recognize that other types ofrefrigeration cycles may be used. Therefore, embodiments of theinvention are not intended to be limited to the particular refrigerationcycles shown and described within the detailed specification and in theaccompanying figures. Additionally, while the embodiments shown in thefigures and discussed herein, typically show that the natural gasexpansion turbine can be connected to a natural gas booster, certainembodiments of the invention are not intended to be so limited. Rather,in certain embodiments of the invention, the natural gas expansionturbine 121 can drive a booster that is located within one of therefrigeration cycles, for example the nitrogen refrigeration cycle. Inthis embodiment, the booster can be configured to compress arefrigeration fluid (for example, nitrogen) within the refrigerationcycle.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims. The presentinvention may suitably comprise, consist or consist essentially of theelements disclosed and may be practiced in the absence of an element notdisclosed. Furthermore, if there is language referring to order, such asfirst and second, it should be understood in an exemplary sense and notin a limiting sense. For example, it can be recognized by those skilledin the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing (i.e.,anything else may be additionally included and remain within the scopeof “comprising”). “Comprising” as used herein may be replaced by themore limited transitional terms “consisting essentially of” and“consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, makingavailable, or preparing something. The step may be performed by anyactor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

All references identified herein are each hereby incorporated byreference into this application in their entireties, as well as for thespecific information for which each is cited.

We claim:
 1. A method for the production of liquefied natural gas(“LNG”) and liquid nitrogen (“LIN”), the method comprising the steps of:a) providing a nitrogen refrigeration cycle, wherein the nitrogenrefrigeration cycle is configured to provide refrigeration within a heatexchanger, wherein a portion of the nitrogen within the nitrogenrefrigeration cycle is withdrawn and liquefied yielding a liquidnitrogen product, wherein at least an equal portion of gaseous nitrogenis introduced to the nitrogen refrigeration cycle as is withdrawn; b)purifying a first natural gas stream in a first purification unit toremove a first set of impurities to produce a purified first natural gasstream; c) cooling and liquefying the first natural gas stream in theheat exchanger using the refrigeration from the nitrogen refrigerationcycle to produce an LNG stream, wherein the first natural gas stream hasan LNG refrigeration requirement, wherein the LNG stream is liquefied ata first pressure P_(H); d) purifying a second natural gas stream in asecond purification unit to remove a second set of impurities to producea purified second natural gas stream; e) partially cooling the secondnatural gas stream in the heat exchanger; f) withdrawing the partiallycooled second natural gas stream from an intermediate section of theheat exchanger; g) expanding the partially cooled second natural gasstream to a medium pressure P_(M) in a natural gas expansion turbine toform a cold natural gas stream, wherein the medium pressure P_(M) is ata pressure lower than the first pressure P_(H); and h) warming the coldnatural gas stream in the heat exchanger by heat exchange against thefirst natural gas stream to produce a warm natural gas stream at thewarm end of the heat exchanger, wherein the natural gas expansionturbine drives a first booster, wherein the LNG refrigerationrequirement is supplied by a combination of refrigeration from thenitrogen refrigeration cycle and step h) Error! Reference source notfound.
 2. The method as claimed in claim 1, wherein the first booster isconfigured to compress the second natural gas stream or a stream derivedfrom the second natural gas stream.
 3. The method as claimed in claim 1,wherein the first booster is configured to compress a stream selectedfrom the group consisting of the first natural gas stream, the purifiedfirst natural gas stream; the second natural gas stream, the purifiedsecond natural gas stream, the partially cooled natural gas stream, thewarm natural gas stream, and a nitrogen fluid within the nitrogenrefrigeration cycle.
 4. The method as claimed in claim 1, wherein theliquid nitrogen product has a LIN refrigeration requirement, wherein theLIN refrigeration requirement is supplied by a combination ofrefrigeration from the nitrogen refrigeration cycle and step h).
 5. Themethod as claimed in claim 1, wherein the first set of impurities has afreezing point at or above the liquefaction temperature of methane atthe first pressure P_(H).
 6. The method as claimed in claim 1, whereinthe second set of impurities comprises water.
 7. The method as claimedin claim 1, wherein the nitrogen refrigeration cycle comprises a recyclecompressor, a turbine, a booster and a plurality of coolers, wherein theturbine and booster are configured such that the turbine is configuredto power the booster.
 8. The method as claimed in claim 7, wherein thenitrogen refrigeration cycle further comprises a nitrogen feedcompressor.
 9. The method as claimed in claim 1, wherein the firstnatural gas stream and the second natural gas stream come from the samenatural gas source.
 10. The method as claimed in claim 9, wherein thenatural gas source is a natural gas pipeline having a pressure between15 and 100 bara.
 11. The method as claimed in claim 1, wherein the firstnatural gas stream comes from a first natural gas source, and the secondnatural gas stream comes from a second natural gas source, wherein thefirst and second natural gas sources are different sources.
 12. Themethod as claimed in claim 11, wherein the first natural gas sourcecomprises a natural gas pipeline.
 13. The method as claimed in claim 12,wherein the natural gas pipeline has a pressure between 15 and 100 bara.14. The method as claimed in claim 1, wherein the first purificationunit and the second purification unit are the same unit.
 15. The methodas claimed in claim 1, wherein the first purification unit and thesecond purification unit are separate units, wherein the firstpurification unit is configured to remove at least water and carbondioxide, and wherein the second purification unit is configured toremove at least water.
 16. The method as claimed in claim 1, wherein thenitrogen liquefier further comprises a subcooler.
 17. A method for theproduction of liquefied natural gas (“LNG”) and liquid nitrogen (“LIN”),the method comprising the steps of: a) providing a nitrogenrefrigeration cycle, wherein the nitrogen refrigeration cycle isconfigured to provide refrigeration within a heat exchanger, wherein aportion of the nitrogen within the nitrogen refrigeration cycle iswithdrawn and liquefied yielding a liquid nitrogen product, wherein atleast an equal portion of gaseous nitrogen is introduced to the nitrogenrefrigeration cycle as is withdrawn; b) cooling and liquefying a firstnatural gas stream in a heat exchanger by heat exchange with nitrogenfrom the nitrogen refrigeration cycle to produce an LNG stream, whereinthe LNG stream is liquefied at a first pressure; c) expanding a secondnatural gas stream to a second pressure to produce an expanded naturalgas stream; and d) warming the expanded natural gas stream in the heatexchanger to produce a warmed natural gas stream, wherein step d)provides a portion of the refrigeration used to cool and liquefy thefirst natural gas stream.
 18. The method as claimed in claim 17, whereinthe first natural gas stream comes from a first natural gas source, andthe second natural gas stream comes from a second natural gas source,wherein the first and second natural gas sources are different sources.19. The method as claimed in claim 17, wherein the liquid nitrogenproduct has a LIN refrigeration requirement, wherein the LINrefrigeration requirement is supplied by a combination of refrigerationfrom the nitrogen refrigeration cycle and step d).
 20. The method asclaimed in claim 17, wherein the first natural gas liquefied in step b)is derived from the expanded natural gas stream, wherein the firstpressure and the second pressure are about the same.
 21. A method forthe production of liquefied natural gas (“LNG”) and liquid nitrogen(“LIN”), the method comprising the steps of: a) providing a nitrogenrefrigeration cycle, wherein the nitrogen refrigeration cycle comprisesa recycle compressor, a turbine, a booster and a plurality of coolers,wherein the turbine and booster are configured such that the turbine isconfigured to power the booster, wherein a portion of the nitrogenwithin the nitrogen refrigeration cycle is withdrawn and liquefiedyielding a liquid nitrogen product, wherein at least an equal portion ofgaseous nitrogen is introduced to the nitrogen refrigeration cycle as iswithdrawn; b) providing a high pressure natural gas stream; c) splittingthe high pressure natural gas stream into a first portion and a secondportion; d) cooling and liquefying the first portion of the highpressure natural gas stream to produce an LNG stream; e) providing afirst portion of refrigeration via the nitrogen refrigeration cycle;providing a second portion of refrigeration by expanding the secondportion of the high pressure natural gas; and g) using the first portionof refrigeration and the second portion of refrigeration to achieve thecooling and liquefaction of the first portion of the high pressurenatural gas stream in step d).